Ethylene copolymers are a well-known class of olefin copolymers from which various plastic products are now produced. Such products include films, fibers, and thermomolded articles such as containers and coatings. The polymers used to prepare these articles are prepared from ethylene, optionally with one or more additional copolymerizable monomers. The first PE grades were made in the 1930's using free radical chemistry at high pressures, and they turned out to be fairly easy to form into various articles. Over the years, it was realized that this was due to the fact that these low density polyethylene (LDPE) resins contained highly branched species. When linear forms of LDPE (LLDPE) became available in the 1960's, it was recognized that films made from them were superior to LDPE films in terms of puncture resistance, impact resistance, tear strengths, and clarity, but they also could not be processed as easily on the existing equipment used for LDPE. The recognition that the form and level of long chain branching (LCB) in LDPE was the reason for both the easy processing and the deterioration in physical properties of the films has led to a long-time effort to tailor the LCB to achieve a better balance of processability (e.g., motor load during extrusion, blown film line speed, bubble stability) and performance (e.g., film mechanical strengths, absence of gels). Low density polyethylene (“LDPE”) as produced by free radical polymerization consists of many LCB structures where the LCB branches occur randomly along the polymer backbone and branches. These structures allowed LDPEs to have easy processing, which means that LDPE can be melt processed in high volumes at low energy input in an extruder, a blown film line, a blow molder, and other polymer processing/fabrication equipment. Since LDPE was the first PE introduced, machinery for conducting PE melt processing, for example extruders and film dies of various configurations, was designed based on the processing characteristics of the LDPE.
However, with the advent of effective coordination catalysis of ethylene copolymers, the degree of long chain branching was significantly decreased, both for the now traditional Ziegler-Natta ethylene copolymers and for the newer metallocene-catalyzed ethylene copolymers. Both, particularly the metallocene copolymers, are essentially linear polymers (but these processes can also lead to branched resins by macromonomer reincorporation), which are more difficult to melt process, especially when the molecular weight distribution (MWD=MW/MN, where MW is weight-average molecular weight and MN is number-average molecular weight) is narrower than 3.5. Although broad MWD copolymers are more easily processed but can lack desirable solid state attributes otherwise available from the metallocene copolymers. Thus it has become desirable to develop effective and efficient methods of improving the melt processing of olefin copolymers while retaining desirable melt properties and end-use characteristics.
The introduction of long chain branches into substantially linear olefin copolymers has been observed to improve processing characteristics of the polymers. Such has been done using metallocene polymers where significant numbers of olefinically unsaturated chain ends are produced during the polymerization reaction. See e.g., U.S. Pat. No. 5,324,800. The olefinically unsaturated polymer chains can become “macromonomers” (or “macromers”) and, apparently, can be re-inserted with other copolymerizable monomers to form the branched copolymers. International publication WO 94/07930 addresses advantages of including long chain branches in polyethylene from incorporating vinyl-terminated macromers into polyethylene chains where the macromers have critical molecular weights greater than 3,800, or, in other words contain 250 or more carbon atoms. Conditions said to favor the formation of vinyl terminated polymers are high temperatures, no co-monomer, no transfer agents, and a non-solution process or a dispersion using an alkane diluent. Increased temperature during polymerization is also said to yield β-hydride eliminated product, for example, while adding ethylene so as to form an ethylene “end cap”. This document goes on to describe a large class of both monocyclopentadienyl and biscyclopentadienyl metallocenes as suitable in accordance with the disclosure when activated by either alumoxanes or ionizing compounds providing stabilizing, non-coordinating anions.
U.S. Pat. Nos. 5,272,236 and 5,278,272, herein incorporated by reference in their entirety, describe “substantially linear” ethylene polymers which are said to have up to 3 long chain branches per 1000 carbon atoms. These polymers are described as being prepared with certain monocyclopentadienyl transition metal olefin polymerization catalysts, such as those described in U.S. Pat. No. 5,026,798. The copolymer is said to be useful for a variety of fabricated articles and as a component in blends with other polymers.
EP-A-0 659 773 describes a gas phase process using metallocene catalysts said to be suitable for producing polyethylene with up to 3 long chain branches per 1000 carbon atoms in the main chain, the branches having greater than 18 carbon atoms.
Reduced melt viscosity polymers are addressed in U.S. Pat. Nos. 5,206,303 and 5,294,678. “Brush” polymer architecture is described where the branched copolymers have side chains that are of molecular weights that inhibit entanglement of the backbone chain. These branch weight-average molecular weights are described to be from 0.02-2.0 MeB, where MeB is the entanglement molecular weight of the side branches. Though the polymers illustrated are isobutylene-styrene copolymers, calculated entanglement molecular weights for ethylene polymers and ethylene-propylene copolymers of 1,250 and 1,660 are provided.
Comb-like polymers of ethylene and longer alpha-olefins, having from 10 to 100 carbon atoms, are described in U.S. Pat. No. 5,475,075. The polymers are prepared by copolymerizing ethylene and the longer alpha-olefins which form the side branches. Improvements in end-use properties, such as for films and adhesive compositions, are taught.
The effect of the LDPE content on the crystallinity and strain hardening of LDPE/LLDPE blends was investigated in “Elongational rheology of LLDPE/LDPE blends”, Journal of Applied Polymer Science, (2003), 88(14). Three LLDPEs (octene, hexene and butene co-monomer) and three LDPEs (melt flow index (MFI) 0.3, 0.9 and 2.5) were used. The blends were prepared using a single screw extruder. The elongational behavior of the blends and their constituents were measured at 150° C. using a RME rheometer. For the elongational viscosity behavior, no significant differences were observed for the strain hardening of the blends containing 10-30% LDPE. Thermal analysis indicated that at concentrations of up to 20% LDPE, there is no significant effect on the melting and crystallization temperatures of LLDPE. The crystallinity and rheology results indicate that 10-20% LDPE is sufficient to provide improved strain hardening in LLDPE.
In “The Effect of Molecular Structure on the Extensional Melt Rheology of Conventional and Metallocene Polyethylenes”, Annual Technical Conference—Society of Plastics Engineers, (2000), 58th (Vol. 1), 1096-1100, extensional melt rheology and processing characteristics of conventional high pressure low density polyethylene (LDPE) and Ziegler-Natta linear low density polyethylene (LLDPE) are compared with both narrow and broad molecular weight distribution (MWD) and long-chain-branched (LCB) metallocene polyethylenes. The effects of MWD and LCB on the melt behavior of these different types of polymers are presented in terms of their dynamic linear viscosities and their strain-hardening behavior from transient tensile stress growth experiments. Film processability properties are also discussed.
In “Rheological Properties and Molecular Structure of Polymer Melts”, Soft Matter, (2011), 7(6), 2273-2283, an overview is given on relations between some features of the molecular structure of polymers and their viscous and elastic properties in the molten state. Moreover, it is discussed how the elongational behavior of polymer melts can be affected by molecular parameters. Besides the effects of molar mass and molar mass distribution, more recent results on the influence of long-chain branching on rheological properties are presented. While relationships between rheology and processing are addressed only briefly, the application of the rheological investigations with respect to a molecular characterization and to the detection of small amounts of long-chain branches, in particular, is discussed. Although obtained on materials of a rather simple molecular structure, it is said that the results described in this short review have the potential to be used for other polymeric materials in the field of soft matter, too.
In “Linear Viscoelastic Model for Elongational Viscosity by Control Theory”, Rheologica Acta, DOI: 10.1007/s00397-011-0598-2, flows involving different types of chain branches were modeled as functions of the uniaxial elongation using generated constitutive model and molecular dynamics for linear viscoelasticity of polymers. Previously, control theory was applied to model the relationship between the relaxation modulus, dynamic and shear viscosity, transient flow effects, power law and Cox-Merz rule related to the MWD by melt calibration. Temperature dependencies and dimensions of statistical chain tubes were also modeled. The procedure presented is said to be very effective at characterizing long-chain branches, and also in providing information on their structure and distribution. Accurate simulations of the elongational viscosities of low-density polyethylene, linear low-density polyethylene and polypropylene, and new types of MWDs were presented. Models were also presented for strain-hardening that included the monotonic increase and overshoot effects.
In “Rheology of Metallocene-Catalyzed Polyethylenes—The Effects of Branching”, Annual Technical Conference—Society of Plastics Engineers, (1999), 57th (Vol. 1), 1200-1204, the shear and extensional rheology of three polyethylenes (PE's) synthesized using metallocene catalysts are compared. One of the PE's is linear, i.e., no long-chain branches (LCB), while the other two have different amounts of long chain branching. The shear viscosity of the linear PE is reflective of the narrow molecular weight distribution of metallocene catalyzed PE's, while the apparently branched PE's exhibit a higher viscosity and an earlier onset of shear thinning. The linear polymer exhibited lower activation energy than the branched PE with similar MW. The linear PE does not show stress-strain hysteresis while the branched polymer does.
In “Shear and Extensional Rheology of Sparsely Branched Metallocene-Catalyzed Polyethylenes”, Journal of Rheology (New York), (2000), 44(5), 1151-1167, a study was undertaken to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index of 1.0. Two Dow INSITE MCPEs with apparently varying levels of LCB (of approximately 0.17 and 0.57/10 000 carbon atoms and one Exxon EXXPOL MCPE with no LCB) were studied. The breadth of distribution as determined by Mw/Mn of the three samples was 2.11 for the Exxon and one of the Dow samples, and 2.42 for the other Dow sample that had the highest degree of LCB. The MCPE with the highest branching seemed to have a slightly higher molecular weight tail in the distribution. The differences in Mw could not account for the appreciably higher zero-shear viscosities of the branched samples relative to the linear sample. Despite the differences in Mw and LCB content in the two Dow samples, they exhibited almost identical shear flow curves at temperatures between 120-170° C. Under constant extension rate deformation, the two samples with LCB showed a significant degree of strain hardening relative to the linear sample. Comparison between the two Dow samples revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior.
In “Quantitative Analysis of Melt Elongational Behavior of LLDPE/LDPE Blends”, Rheologica Acta, (2004), 44(2), 198-218, shear and elongational data of blends of linear (LLDPE) and branched (LDPE) polyethylene are reported. Blends show thermo-rheological complex behavior. Also, in unidirectional shear or elongational flow, the linear-viscoelastic deformation regime of the blends is significantly reduced, and the terminal relaxation times of the blends are shifted in the direction of the LDPE homopolymer. Quantitative analysis of elongational viscosity data by use of the Molecular Stress Function (MSF) model reveals that the strain hardening behavior of LLDPE/LDPE blends is completely determined by the LDPE component.
In “Influence of Molar Mass Distribution and Long-Chain Branching on Strain Hardening of Low Density Polyethylene”, Rheologica Acta, (2009), 48(5), 479-490, low-density polyethylenes (LDPE) were synthesized in a laboratory scale autoclave under high pressure. These samples were found to possess a high molar mass tail, resulting in a distinctly bimodal molar mass distribution and a lower concentration of long-chain branching than typical of commercial LDPEs. Rheological experiments in elongation showed that these samples exhibited a pronounced strain hardening, which could be favorable for distinct processing operations. Although the samples have a rather high molar mass, their zero shear-rate viscosities and shear thinning behavior are still in a range where thermoplastic processing is possible.
In “Shear Modification and Elongational Behavior of Two Types of Low-Density Polyethylene Melts with Different Long Chain Branching”, AIP Conference Proceedings, (2008), 1027(Pt. 1, 15th International Congress on Rheology, 2008), 454-456, transient biaxial elongational viscosity was measured by the lubricated squeezing flow method in constant volume condition for two kinds of low density polyethylenes (LDPE) with different long chain branches. Significant strain-hardening was observed at low strain rates for both LDPEs. In addition, strain-hardening of both LDPEs was comparable although strain-hardening in uniaxial elongation exhibits great differences. The results are said to be possibly explained by the effect of flow history on elastic properties of LDPEs.
In related patents, “Processing Olefin Copolymers”, U.S. Pat. Nos. 6,355,757; 6,391,998 and 6,417,281, each of which are herein incorporated by reference in their entirety, Garcia-Franco et al., there are disclosed branched hydrocarbon copolymers described as having a main, or backbone, chain of ethylene and other insertion copolymerizable monomers, containing randomly distributed side chains of ethylene and other insertion copolymerizable monomers. The backbone chain has a number-average molecular (MN) weight of from 5,000 to 1,000,000 Daltons, preferably from 10,000 to 500,000 Daltons, most preferably from 20,000 to 200,000 Daltons. The side chains have number-average molecular weights of from 2,500 to 125,000 Daltons, preferably from 3,000 to 80,000 Daltons, most preferably from 4,000 to 60,000 Daltons
While long-chain-branching (LCB) technology is well-known as the method of choice to improve the processability of polymers and has been part of the polyethylene (PE) industry since its inception in the 1930's, the resulting improved processability is nearly always accompanied by a loss in the mechanical performance of the material. An effective, inexpensive additive that would have a significant impact on the processing/performance balance for such polymers should be useful in a large fraction of the multi-billion dollar market for polyethylene films and molded articles. There could even be a larger impact on polypropylene, where there is very little commercially viable technology for incorporating LCB. This would also be useful in the EPDM elastomer market.
It is an object of the present disclosure to provide additives for substantially linear poly(alpha-olefins), such as polyethylene and polypropylene, and other polymers, such as EPDM elastomers, which provides a positive impact on the processing of such polymers and the overall performance balance for such polymers.
It is also an object of the present disclosure to provide compositions comprising substantially linear poly(alpha-olefins), such as polyethylene and polypropylene, and other polymers, such as EPDM elastomers, and an amount of an additive, which compositions have improved processing and improved overall performance balance for such compositions.
It is also a further object of the present disclosure to provide methods for improving processing of compositions comprising substantially linear poly(alpha-olefins), such as polyethylene and polypropylene, and other polymers, such as EPDM elastomers, as well as improving the overall performance balance for such compositions.
These and other objectives are accomplished and will be understood by reference to the following drawings and detailed description.